1. Field of the Invention
The present invention relates to an ultrasonic diagnostic system wherein information as to the inside of a subject is indicated on the basis of received signals which will be obtained through receiving ultrasonic beams or ultrasounds reflected within the subject.
2. Description of the Related Art
There has been used an ultrasonic diagnostic system in which ultrasonic beams are transmitted to the inside of the subject and the human body particularly, the ultrasounds reflected by a tissue in the human body are received in the form of received signals, and an ultrasonic image within the human body based on the received signals is displayed, thereby serving to diagnose diseases of the viscera and the like of the human body. In one aspect of this ultrasonic diagnostic system, or in an optional function of an ultrasonic diagnostic system for displaying a tomographic image (B-mode), there has been used an ultrasonic Doppler diagnostic system in which ultrasounds reflected by blood cells flowing within the human body are received to obtain blood flow information such as velocity, variance, power and the like of the blood flow.
FIG. 3 is a schematic construction view of one example of an ultrasonic diagnostic system.
Since the basic arrangement of the ultrasonic diagnostic system is well known, there will be explained only an arrangement (an ultrasonic Doppler diagnostic system (see U.S. Pat. No. 5,042,491, U.S. Pat. No. 5,215,093)) involved in the present invention in which information representative of a movement within the subject and a blood flow particularly is extracted.
An ultrasonic probe 1 shown in FIG. 3 is constituted of a plurality of ultrasonic transducers 9 toward which pulse signals are each transmitted from a transmitting and receiving circuit 2 in an associated predetermined timing, so that the ultrasonic probe 1 transmits ultrasonic pulse beams to the inside of the subject 30. In this case, for example, a sector scanning is carried out to transmit the ultrasonic pulse beams, for instance, 8 times along each given scan line 31. The ultrasonic pulse beams transmitted to the inside of the subject are reflected by blood cells flowing within the human body and another tissues and received by the plurality of ultrasonic transducers 9 of the ultrasonic probe 1. The received signals received by the ultrasonic transducers 9 are each passed to the transmitting and receiving circuit 2 to be beamformed so as to obtain a received signal carrying information along a predetermined scan line 31. The received signal, which has been subjected to a beamform process, is fed to a quadrature detector 3 to perform a quadrature detection taking as reference signals a sine signal and a cosine signal wherein a center frequency of the ultrasonic wave is given as a reference frequency. The received signal, which has been subjected to a quadrature detection, is fed to an A/D converter 4 to be converted into a digital signal and then passed to an MTI (Moving Target Indication) filter 5. The MTI filter 5 is similar to an MTI filter used in a radar, and usually may be a digital high-pass filter adapted to cut off a low frequency signal, which comprises a delay circuit providing a delay time equivalent to a repeated cycle of the pulse signals and multiplying/adding device. Such an MTI filter is widely used in the field of the ultrasonic diagnostic system. The MTI filter 5 serves to eliminate a low frequency component of the entered signal, or information as to a motion of a relatively slow speed of tissue within the subject, and extract a signal carrying information (blood flow information) as to a relatively high speed of blood flow.
The signal outputted from the MTI filter 5 is fed to a velocity operating circuit 6 to evaluate a blood flow velocity. Information representative of the blood flow velocity thus obtained is passed to a scan converter 7 to be converted into an indication signal. Such an indication signal is superposed on, for example, a B-mode image produced by a B-mode image producing circuit (not illustrated) and then displayed on a display screen of a CRT 8 with a color for example.
As mentioned above, the ultrasonic received signal includes a component (referred to as "clutter information" hereinafter) obtained through receiving the ultrasonic beams reflected by tissues and a component (blood information) obtained through receiving the ultrasonic beams reflected by blood cells. Usually, the clutter information has a power (the order of 20dB-50dB) which is extremely larger than that of the blood information. Consequently, assuming that the A/D converter 4 produces, for example, 10 bits of digital signals, the blood information will be represented by 2 or 3 bits of 10 bits. The digital signals outputted from the A/D converter 4 is fed to the MTI filter 5 to eliminate the clutter information, so that the blood information represented by 2 or 3 bits is extracted. This involves low resolution of blood information. Therefore, it will bring about a problem such that a precision of the velocity of the blood flow evaluated in the velocity operating circuit 6 is very bad. The bad precision of the velocity of the blood flow causes such a situation that particularly in peripheral blood vessels or the like, which are small in a received signal level, the blood flow is not extracted in spite of the presence of the blood flow. Thus, it happens that the blood flow is not displayed on the CRT 8.
In order to enhance the precision, it is considered that a bit length in the A/D converter 4 is increased. This will involve, however, an enlargement of the scale of the A/D converter 4, the succeeding MTI filter 5, the velocity operating circuit 6 and the like. Thus, it brings about such a problem that the system is obliged to increase remarkably the cost for fabrication.
In view of the foregoing, it is an object of the present invention to provide an ultrasonic diagnostic system capable of extracting information as to motion within the subject, without enlarging a bit length of the A/D conversion.